The present invention relates to hydraulic valve lifters for use in reciprocating internal combustion engines, and, more in particular, to a hydraulic valve lifter having a controlled oil leak path to reduce the lift of the lifter at low engine speeds while permitting full lift at some higher engine speed for maximum inlet and exhaust valve overlap, valve lift and valve duration. Leakage also lubricates the surface of the lifter and a cam which contact each other.
A reciprocating internal combustion engine has a number of cylinders, each of which has an inlet valve and an exhaust valve for admitting a combustible mixture and exhausting products of combustion, respectively. Pistons in the cylinders are agents for inducting combustible mixture into the cylinders by suction, and for forcing exhaust products from the cylinders by displacement. In a cylinder of a modern engine, an inlet valve begins to open while an exhaust valve is still open, but closes during the time that the piston of the cylinder is ending an upward stroke and beginning a downward one. When both exhaust and inlet valves are open at the same time there is valve overlap.
Today's focus on engine emission problems is well known. The pollutants of most concern are unburned hydrocarbons, the oxides of nitrogen (NO.sub.x), and carbon monoxide.
In the general case, unburned hydrocarbon and carbon monoxide pollutants are reduced with lower valve overlap and valve open durations. The reasons for this favorable result include a longer duration of time during a cycle that the combustion chambers are closed and, therefore, a longer time for completing combustion at higher average combustion temperatures. At higher temperatures, combustion rates are faster. The longer the time and the faster the rate, the more complete the combustion.
In the general case, with valve overlap and at normally encountered engine speeds, higher pressure in the exhaust side of an engine over the pressure in the induction or inlet side of the engine will force exhaust products back into a combustion chamber. This lowers combustion temperatures because the presence of exhaust products precludes the presence of a corresponding amount of combustible mixture. With lower combustion temperatures, oxides of nitrogen generated during the combustion process are reduced because the reaction rate of the reactants which produce the oxides decreases dramatically as temperature is lowered.
While valve overlap has advantages at relatively high engine speeds, it generally adversely affects engine operation at low engine speeds, pollution considerations aside.
To accommodate the different operating conditions of an engine and to get satisfactory engine performance throughout the engine's speed range, elaborate mechanisms have been proposed for varying the degree of valve overlap between a condition where extremely low overlap occurs at low engine speeds to a condition where the overlap is greater at high engines speeds. It is recognized that the proposed systems are elaborate, complex, and require considerable development. See Freeman and Nicholson, Valve Timing for Control of Oxides of Nitrogen (NO.sub.x), Society of Automotive Engineers, Paper 720121 (1972). It has been known for some time that a large amount of valve overlap at high engine speeds dramatically increases an engine's power performance. Large valve overlap admits to a greater mass of fuel and air in a combustion chamber at operating conditions where the inertia of exhausting products of combustion overcomes the effect of exhaust gas pressure to keep products of combustion from reentering the combustion chamber from the exhaust system. In this high performance application it has also been recognized that large valve overlap produces very poor to totally unsatisfactory idle performance and poor driveability. In this application, proposals have been made to use hydraulic valve lifters as a vehicle for varying the amount of valve overlap as a function of engine speed (from a condition of low overlap to high overlap with increasing engine speed). In U.S. Pat. No. 3,304,925 to Rhodes, for example, a hydraulic valve lifter is disclosed having two chambers communicating through a check valve with oil in one of the chambers being in series force relationship in the valve train, as is standard in hydraulic valve lifters. Rhodes' valve lifter provides a controlled release of pressure from this chamber to a channel between the lifter's plunger and lifter body. This release of pressure occurs only above a certain threshold engine speed and diminishes gradually thereafter with increasing engine speed to increase valve lift and valve overlap. In U.S. Pat. No. 2,614,547 to Meinecke a hydraulic valve lifter is disclosed which provides increasing valve overlap with increasing engine speed by gradually increasing the oil pressure in a chamber of the valve lifter in which oil is in series force relationship in the valve train. At low engine speeds the Meinecke valve lifter discharges oil from the chamber externally of the valve lifter. The same type of arrangement is disclosed in U.S. Pat. No. 2,931,347 to Williams.